Method and device for measuring the thickness of a metal deposit on an insulating support

ABSTRACT

Plastic film is drawn from a spool and passes around a drum in a vacuum chamber in such a way that the film is exposed to metal vapor which is deposited thereon in thickness ranging from 50 angstroms to several microns. The metallized film then passes around guide rollers and is wound on a reel. One of the guide rollers comprises a novel hermetically sealed shell of electrical insulating material containing novel detectors which are responsive to the thickness of the metal coating on the film. Such detectors consist of coreless induction coils, each constituting the tuning element of a unique stabilized oscillating circuit, the voltage of which is a very accurate measure of the relative thickness of the metal coating on the film by virtue of the eddy currents induced therein.

United States Patent Commercon et al.

[ 1 July 25,1972

[54] METHOD AND DEVICE FOR MEASURING THE THICKNESS OF A METAL DEPOSIT ONAN INSULATING SUPPORT [72] Inventors: Jean Claude Commercon, 35 Chemindes Petites Brosses, 69 Caluire; Guy Berthier, Avenue Posteur, 01 SaintMaurice de Beynost, both of France [22] Filed: March 16, I970- [211App]. No.: 19,933

[52] U.S. Cl ..324/34 TK [5]] Int. Cl....

[58] Field of Search [56] References Cited UNITED STATES PATENTS3,077,858 2/1963 Ulug ..324/34 TK 2,545,576 3/1951 Godley 2,676,2984/1954 Frommer ..324/34 TK DIFF.

DET. SOURCE FOREIGN PATENTS OR APPLICATIONS 1,083,231 9/1967 GreatBritain ..324/34 TK Primary Examiner-Rudolph V. Rolinec AssistantExaminer-R. J. Corcoran Attorney-M. Smolowitz [5 7] ABSTRACT Plasticfilm is drawn from a spool and passes around a drum in a vacuum chamberin such a way that the film is exposed to metal vapor which is depositedthereon in thickness ranging from 50 angstroms to several microns. Themetallized film then passes around guide rollers and is wound on a reel.One of the guide rollers comprises a novel hermetically sealed shell ofelectrical insulating material containing novel detectors which areresponsive to the thickness of the metal coating on the film. Suchdetectors consist of coreless induction coils, each constituting thetuning element of a unique stabilized oscillating circuit, the voltageof which is a very accurate measure of the relative thickness of themetal coating on the film by virtue of the eddy currents inducedtherein.

3 Claims, 2 Drawing Figures 36 Regulating {Q 64 eons R AMPLIFIER \66 (UM (0/ Recording N 0 ll 62 Device (AC) AMPLIFIER 2 l5 /4 26 22 20 RECT. 2H

METHOD AND DEVICE FOR MEASURING THE THICKNESS OF A METAL DEPOSIT ON ANINSULATING SUPPORT This invention relates to the continuous productionof metallized plastic film and more particularly to the art of measuringthe thickness of a metal deposit on an insulating support.

The principle is known which consists in measuring the variations incurrent of an induction circuit which causes the appearance of eddycurrents in a continuously metallized sheet, the amount of whichcurrents is a function, inter alia, of the thickness of themetallization layer and the presence of which causes measurable lossesin the induction circuit.

It is also known to form a free running oscillator the coil of which iscoupled with the metallized layer, measuring the variations in frequencyunder the influence of the reactance reflected in the coil by themetallized film. Such measurement gives results which can be suitablewhen large thicknesses are measured; but its degree of precision isinsufficient for small thicknesses of the order of a few microns, as istrue of metallized films.

By connecting the induction coil to a source of highfrequency voltagemodulated by a substantially constant amplitude, and connecting it to acapacitor, one can also create a resonant circuit which is charged bythe reactance reflected in the coil by the metallized film, causingvariations of amplitude and frequency in the circuit which form ameasure of the thickness of metal deposited. Such method, which is ofcomplicated technique, also lacks sensitivity for slight thicknesses.

The known methods used up to the present time have drawbacks which limittheir interest. Some of them employ the change of frequency of apiezoelectric crystal but such method takes into account only whathappens locally, which limits the quality of the information in case ofmetallization in large width. Other methods effect the directmeasurement by means of electrodes of the surface electric resistance,but the production of proper electric contacts has always left much tobe desired, particularly at high speeds of travel. One proceeds also bymeasuring the optical opaqueness of the metallized films, but suchmethod is relatively insensitive on both sides of a small range ofthicknesses of deposit; it furthermore requires sensitive apparatus onboth sides of the film, and the measures are rapidly falsified by thecondensation of metallic vapors on the instruments.

The main object of the present invention is to provide apparatus whichavoids such drawbacks, which does not involve direct contact with thefilm, the vital members of which are protected from the metal vapors,which can measure the thickness of thin layers or thick layers, which isalso suitable in the case of double-sided metallization, both on smalland on large widths. This invention permits the control of one or moreoperating parameters of a continuous metallization machine as a functionof the thickness of the layer measured. It makes it possible also tomeasure both the opaqueness of the metal deposit and its resistance perunit of surface or its thickness.

According to the invention there is provided a free running oscillatorwhose coreless detector coil, which gives rise to eddy currentsin thecontinuously metallized film constitutes the tuning element, the inputvoltage of the oscillating circuit being stabilized by a suitabledevice, and measuring the overvoltage caused in this induction circuitby the variations in the induced current resulting from variations inthickness of the deposited metal. This makes it possible to carry outcorrect, faithful, stable measurements within a range of square ohmicsurface resistance extending from a few hundredths of an ohm to about athousand ohms, which corresponds, depending on the materials, tothicknesses ranging from about 50 angstroms to several microns.

The invention comprises measuring a factor Q of an oscillating circuitconsisting of a source of voltage, of an induction coil (L), and of acapacitor (C). The factor Q of the circuit in which (L) is theself-inductance of the induction coil and (C) is the capacitance of thecapacitor is a function of (L), (C)

and the characteristics of the probe, and is measured by the ratio Uc/e,Uc being the voltage at the terminals of the capacitor and e the voltagegiven off by the voltage source. If one maintains e constant, the factorQ will vary as a function ofthe load of the induction coil.

The latter, due to the currents induced in the conductive layer, behavesin fact as the primary of a perfect transformer of which the metallizedfilm is the secondary. This load in accordance with the classical theoryof perfect transformers, can be considered equivalent to a fictitiousseries resistance in the circuit of the coil, which accordingly modifiesthe overvoltage coefficient which is represented by factor Q. Anyvariation of the thickness of the metal deposit which will result in avariation of the currents induced within the layer will, therefore,

produce an equivalent variation of the fictitious resistance of theprimary of the transformer formed, as stated, of the coil and film andtherefore, of the overvoltage coefficient Q of this circuit.

IN THE DRAWING FIG. 1 is a circuit diagram illustrative of theinvention.

FIG. 2 is a flow diagram of a plastic film metallizing and metal coatingthickness measuring control system incorporating the invention.

As shown in FIG. 1, the circuit 10 includes a variable-gain amplifier 12of conventional type, servo-controlled, having relatively lowimpedances. An oscillating circuit 14 composed of a coreless inductioncoil 16 and its tuning capacitor 18 is connected to the amplifier 12 andground -20. The oscillator return loop 22 includes a capacitor 24 ofvery low impedance compared to that of capacitor 18, making negligiblethe value of the resistance introduced in series with coil 16, whilemaintaining the desired oscillation in the system. A control circuit 26is provided comprising a stable-gain amplifier-rectifier 28, a highlystabilized voltage (e) source 30, and a difference (E) detector 32. Thelatter produces a control signal which is applied at R in order tocontinuously modify the gain of amplifier l2 and stabilize the voltageat O.

Another amplifier 34 of high input impedance is connected at N tocircuit 22 and associated with a direct current voltmeter 36 whichserves to indicate-the voltage U0) and therefore the factor 0 since thevoltage at O is kept constant.

In this apparatus, the induction coil 16 constitutes a detector elementshown in 44, FIG. 2 and is provided with no core, since the presence ofa core while increasing the value of the factor Q, would decrease thetemperature stability and therefore, the sensitivity of the apparatus.The dimensions and number of turns of the coil 16 are determined by theconditions of use, such as value of the resistance, sensitivity andoperating frequency.

The stabilized voltage at O constitutes a high-frequency feed, theprecision of the stabilization of which is a function essentially of thethermal stability; it must be of the order of 10* at 23C 1. Theoperating frequency can be between (Hz and l0 MHz per second, the choiceof the frequency being determined by the necessity of eliminatingparasitic influences, by the desired sensitivity which is a function ofthe frequency, and by conveniences of construction.

The calibrating of the device 10 is effected by a study of the discretevalues of the reactions between the induction coil 16 and the metallizedlayer, which makes it possible for instance, with the assistance of acomputer, to plot a calibration curve of the factor Q as a function ofthe resistance per unit of surface.

The detection elements 16 are arranged within a hermetic, hollowinsulating roller 38, FIG. 2, which turns freely under the effect of thepassage of a film 40 freshly metallized by the technique of vacuummetallization.

In accordance with this technique, a vacuum chamber 42 is providedwithin which there is arranged a film supply roller spool 44 of theplastic film 40 to be metallized. The film 40 passes around ametallization drum 46 and tnen, after passage arranged in the enclosure.

around the guide roller 68 and roller 38, is rewound on reel 50. In thelower part of the enclosure 42 there is arranged a crucible '52 heatedby a resistor 54 in which a metal is evaporated. The metallizationvapors 56, guided by a diaphragm 58, condense on the film 40 at the drum46. A screen 60 has the effect of preventing metal condensationsoccurring elsewhere than atthe desired place on the other parts detector16 is arranged at a small distance of about 1 cm from the unwindingfreshly metallized film 40. These detectors 16 are connected by means ofconnections arranged in the hollow axis of the roller which containsthem, to the amplifiers and other elements which constitute the devicein accordance with the invention. Any possiblevariations in temperaturewhich may disturb the measurements are combatted by thermostating theenclosure or else by automatic variation of the characteristics of thecomponents.

The plotting of calibration curves verified by the measurement of theoptical densities makes it possible to show that the variations in thefactor Q are closely and precisely related to the variations in ohmicresistance which themselves are a function of the variations inthickness of the metallization layer.

, The precise plotting of these; calibration curves makes it possible todefine the index instruction values of the factor Q adapted to assureprecisecharacteristics of the layer in accordance with thespecificationsof the applications contemplated. i

Known control devices of any kind can be controlled by the apparatusdescribed,- among which are a device 62, FIG. 1, for

' the graphical recording of the measured variations of the overvoltage,an .alarm 64 for emitting an alarm signal when the characteristics moveoutside fixed tolerances, or regulating means 66, the error voltagebetween the average index value selected and the instantaneous valuecontrolling, for instance,

the speed of passage, the power of evaporation of the metal, or anyother manufacturing parameter. The invention makes it possible inparticular instantaneously to control the start of the metallizationoperation'and therefore, to limit the manufacturing waste and assure theproper homogeneity thereof.

In operation, plastic film 40 is drawn from spool 44 and passes underdrum 46 in the vacuum chamber 42, so that the v film 40 is exposed tothe metal vapor 56 which is deposited thereon in thickness ranging from50 angstroms to several microns. The metallized film 40 then passesabove guide roller 68, under roller 38 and is finally wound on the reel50. The reel 50 is driven by a motor 70, the speed of which is under thecontrol of the regulating means 66. The detectors 16 in the roller 38cause the circuit to be responsive to the thickness of the 'metaldepositon the film 40 by virtue of the eddy currents induced therein, and thespeed of the reel 50 is charged accordingly by the operation. ofregulating means 66 with respect to reel drive motor 70. For example,the speed of film 40 is automatically increased when the metal layertends to increase in thickness; or decreased when the metal film 40tends to decrease; whereby the thickness of the metal layer on the film40 is kept constant.

In this apparatus, the detector consists of an induction coil 16 withoutcore, since the presence of a core, while increasing the value of thefactor Q, would decrease the temperature" stability and therefore, thesensitivity of the apparatus. The

dimensions and the number of turns of the coil are predeterelectricallyconductive layer on a continuously moving film,

said apparatus comprising:

an oscillating circuit including a variable-gain amplifier with arelatively low output impedance and having an output connected to aself-induction coil and a tuning capacitor connected in series circuitrelationship, said coil and said capacitor forming the tuning elementsof said oscillating.

circuit, said self-induction coil being disposed with respect to saidconductive layer on said film such that the current which flows in saidself-induction coil develops eddy currents withing the conductivelayer,a vacuum enclosure, 4 b a freely rotating roller of electricallyinsulating material within said enclosure, said'coil being arrangedin'said vacuum enclosure within said freely rotating roller, said rollerbeing in contact with the film after deposition ofthe conductive layerthereon, i i A feed-back loop in said oscillating circuit connected tothe junction between saidcoil and said capacitor and the 0 input of saidvariable gain amplifier and including a second capacitor returned to theinput of said variable gain amplifier for sustaining the oscillations ofsaid oscillating circuit,

a control circuit includinga stable-gain amplifier-rectifier connectedto the output of said variable gain amplifier, a stabilized voltagesource, I a difference voltage detector connected to the output of saidstable-gain amplifier-rectifier and to said stabilized voltage source,the output of said detector being connected to said variable gainamplifier in order to continuously modify the gain thereof and tomaintain its output voltage constant,

a high input impedance amplifier means connected to said feedback loop,

indicating means connected to said amplifier means for indicating theoutput voltage variations of said loop, said output variations beingrepresentative of the relative amount of said eddy currents induced inthe conductive layer by said coil,

the voltage indicated by said indicatingmeans being a mea- 7 sure of thethickness of the conductive layer on said film,

regulating means connected to the output of said indicatingv means,

and means connected to said regulating means for adjusting the rate ofmovement of said film in response to the output of said indicatingmeans. 2. Apparatus according to claim 2, whereas said indicating meanscomprises a graphic recording device connected to said amplifier meansfor recording said voltage variations.

3. Apparatus according to claim 1, an alarm device whereas saidindicating means further comprises for producing an alarm signal whensaid thickness connected to said amplifier means detected exceedspredetermined limits.

t: s s a i

1. Apparatus for measuring variations of the thickness of anelectrically conductive layer on a continuously moving film, saidapparatus comprising: an oscillating circuit including a variable-gainamplifier with a relatively low output impedance and having an outputconnected to a self-induction coil and a tuning capacitor connected inseries circuit relationship, said coil and said capacitor forming thetuning elements of said oscillating circuit, said self-induction coilbeing disposed with respect to said conductive layer on said film suchthat the current which flows in said self-induction coil develops eddycurrents withing the conductive layer, a vacuum enclosure, a freelyrotating roller of electrically insulating material within saidenclosure, said coil being arranged in said vacuum enclosure within saidfreely rotating roller, said roller being in contact with the film afterdeposition of the conductive layer thereon, A feed-back loop in saidoscillating circuit connected to the junction between said coil and saidcapacitor and the input of said variable gain amplifier and including asecond capacitor returned to the input of said variable gain amplifierfor sustaining the oscillations of said oscillating circuit, a controlcircuit including a stable-gain amplifier-rectifier connected to theoutput of said variable gain amplifier, a stabilized voltage source, adifference voltage detector connected to the output of said stable-gainamplifier-rectifier and to said stabilized voltage source, the output ofsaid detector being connected to said variable-gain amplifier in orderto continuously modify the gain thereof and to maintain its outputvoltage constant, a high input impedance amplifier means connected tosaid feedback loop, indicating means connected to said amplifier meansfor indicatIng the output voltage variations of said loop, said outputvariations being representative of the relative amount of said eddycurrents induced in the conductive layer by said coil, the voltageindicated by said indicating means being a measure of the thickness ofthe conductive layer on said film, regulating means connected to theoutput of said indicating means, and means connected to said regulatingmeans for adjusting the rate of movement of said film in response to theoutput of said indicating means.
 2. Apparatus according to claim 2,whereas said indicating means comprises a graphic recording deviceconnected to said amplifier means for recording said voltage variations.3. Apparatus according to claim 1, an alarm device whereas saidindicating means further comprises for producing an alarm signal whensaid thickness connected to said amplifier means detected exceedspredetermined limits.